Certified GKH Gardening Expert
This could be a number of things - too little water (especially if something is restricting the roots), over fertilizing, leaf scorch (if planted in too much sun), or even a fungus. It may help to prune the plants to help rejuvenate them. Also, step on the soil around the plants to eliminate any air pockets that may be lingering, which would make it difficult for the plants to take up water, then water the area well. And to err on the safe side, treat them with fungicide.